The development of in-fiber Bragg gratings (FBGs) has led to their use in wavelength measuring systems for sensor and telecommunication systems as well as for wavelength division multiplexing, dispersion compensation, laser stabilization and erbium gain flattening, all around 1550 nanometer wavelengths. These applications depend on FBG wavelength references which either do not change with temperature or change in a predictable manner.
The temperature coefficients of thermal response characteristics of FBGs are unacceptably high when the FBGs are used as wavelength reference or wavelength-selective passive component. Control of the thermal response characteristics not only enable FBGs to have low temperature coefficients, but also allow the temperature coefficients of FBGs to be tailored to match, or track, the temperature coefficients of other components used in an optical wavelength reference system. For example, the FBG can be used as a marker to identify a particular wavelength in the comb of wavelengths produced by a fiber Fabry Perot filter when the wavelengths of the comb drift with temperature.
Accurate wavelength referencing requires either temperature calibration or temperature compensation, or else temperature control, of the wavelength reference devices. In the latter case, additional sensing and control circuitry as well as thermoelectric heater/coolers requiring high electrical power consumption are generally needed. The resulting devices are usually not compact and robust nor maintenance-free for long-term applications. Temperature compensation or calibration is the more practical technique, with compensation being preferred since it provides an independent reference with a simpler and maintenance-free construction which requires no correction calculations circuitry.
Various methods have been devised for achieving temperature independence for the wavelengths of FBGs. These methods range from active systems that utilize feedback to monitor and dynamically control certain parameters, to passive devices that utilize the thermal characteristics of materials/structures to modify the response of the FBG wavelength to temperature. Passive devices are more desirable since they are much simpler and require no power source and so are generally maintenance-free. The wavelength of an FBG is determined by the refractive index of the fiber and the spacing of the grating, both of which change with temperature. Since the refractive index is not easily controlled, passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG This is usually accomplished by clamping the fiber containing the FBG onto a mechanical structure, which is designed to result in a compression of the fiber with increasing temperature.
G. W. Yoffe et al. proposed to use the differential thermal expansion of a silicon tube and an aluminum tube. The optical fiber was glued to the aluminum tube using epoxy. A nut on the threaded aluminum tube adjusts for fiber pre-tension. A wavelength shift of 0.7 pm/° C. was achieved, but the overall structure requires precision-made components and is complicated to assemble (G. W. Yoffe, P. A. Kurg, F. Ouellette, and D. A. Thomcraft, “Temperature-compensated optical fiber Bragg gratings,” in Optical Fiber Communications, vol. 8 of 1995 OSA technical Digest Series (Optical Society of America, Wash., D.C., 1995) pp. 134–135). More complicated design based on similar principle to offer temperature compensation over a wider temperature range has been patented by Lin et al. (Lin et al., “Temperature-compensating device with tunable mechanism for optical fiber gratings,” U.S. Pat. No. 6,374,015, 2002).
Miller et al. achieved temperature compensation by using the bi-material strip of quartz and stainless steel (Miller et al., “Temperature compensated fiber Bragg gratings,” U.S. Pat. No. 6,044,189, 1997). Strip widths of the steel and quartz were varied to achieve the desired level of compensation. The device is much easier to manufacture than Yoffe et al. and Lin et al.'s. However, all the above device are fragile and heavy and may present serious sideway strain to the delicate fiber. T. Iwashima et al. made use of the differential expansion of epoxy filled liquid crystal polymer tube to achieve a temperature coefficient of 1.3 pm/° C. Although the temperature coefficient was inferior to that of Yoffe et al.'s, the overall structure is much simpler to make. Moreover, the latter structure is lighter and more robust than the former devices (T. Iwashima, A. Inoue, M. Shigematsu, M. Nishimura, and Y Hattori, “Temperature compensation technique for fiber Bragg gratings using liquid crystalline polymer tubes,” Electron. Lett., vol. 33, pp. 417–419, 1997).
Beall et al. of Coming Glass Work developed a ceramic that has a negative thermal expansion coefficient. The compensated wavelength shift achieved was 1.212 pm/° C. Very careful control of the formulation of materials is required to obtain the desired negative temperature coefficient of expansion and the ceramic is fragile (Beall et al., “Athermal optical device,” U.S. Pat. No. 6,087,280, 2000).
In 2001, Prohaska et al. proposed to make use of the anisotropic nature of calcite. Such anisotropy will lead to a profile of thermal expansion coefficients along different orientations. Certain orientation can be found to provide the necessary thermal compensation to the wavelength shift of the FBGs. Accurate formulation of materials is required to obtain the desired coefficient profile and precise alignment of fiber along the chosen orientation is needed (Prohaska et al., “Temperature compensated fiber grating and method for compensating temperature variation in fiber grating,” U.S. Pat. No. 6,240,225, 2001).
The passive methods described in the preceding paragraph have the disadvantages of being relatively bulky, heavy, complicated to manufacture, and fragile. Moreover, modification of the degree of thermal compensation can only be done by using complicated mechanisms, massive structure or re-formulation of component material and as a result is difficult and expensive. It is therefore an object of this invention to produce a light-weighted, small-sized, simple, robust and inexpensive device which can provide passive temperature compensation for FBGs and other optical systems. The degree of compensation can be easily and inexpensively designed beforehand.